A numerical model is built for masonry structures by means of discrete element method, with the masonry structure being regarded as a threephase composite material consisting of block, mortar and cohesion in between. The pattern of cracking and deformation of masonry structures under differential settlement induced by tunnel construction is studied on basis of energy theory. The static energy peak index of masonry structures under differential settlement is proposed by means of energy theory analysis. The study results show that the discrete element numerical simulation method can be used to analyze the influence of differential settlement on the safety of masonry structures and that the mentioned method can provide a new approach for the evaluation of the safety of masonry structures disturbed by surrounding construction activities.